BACKGROUND OF THE INVENTION.
[0001] The present invention relates to communications in wireless networks. More specifically,
but not exclusively, the present invention relates to signaling techniques for adaptively
modulating communications in a high throughput wireless network.
[0002] Most communications networks are designed to convey multiple communications simultaneously
over each individual communication path, for example, a radio frequency (RF) channel,
using some type of modulation. In recent years, an increasing demand has arisen for
more efficient and reliable digital data transfers which assure correct data transmissions
at as high a data rate as possible.
[0003] Orthogonal frequency division multiplexing (OFDM) is an increasingly attractive modulation
technique for high-bandwidth wireless applications since it dramatically simplifies
equalization of intersymbol interference (ISI) channels.
[0004] Using link adaptation (LA), it is possible to improve throughput and/or efficiency
in wireless OFDM systems by adjusting transmission parameters, such as subcarrier
modulation orders, power allocation and/or code rate, to best fit the current channel
state.
[0005] Ideally, link adaptation would adapt at every time instant in frequency to the instantaneous
channel realizations. Unfortunately, limitations in feedback bandwidth and variation
of the channel due to Doppler spread make ideal link adaptation difficult to realize.
[0006] One of the difficulties encountered in LA for wireless networks, for example, high
throughput (HT) wireless local area networks (WLANs) with adaptive OFDM, is the useful
duration of channel adaptation information. Since propagation channels can change
rapidly due to Doppler and other effects, the useful duration of channel adaptation
information may be dependent on the coherence time of the channel. Coherence time
is the time domain dual of Doppler spread (i.e., Doppler spread and coherence time
are inversely proportional to one another) and is used to characterize the time varying
nature of the frequency dispersivenes of a channel in the time domain.
[0007] Coherence time is a statistical measure of the time duration over which the channel
impulse response is essentially invariant, and quantifies the similarity of the channel
response at different times. In other words, it is the time duration over which two
received signals have a strong potential for amplitude correlation.
[0008] Network environments with long channel coherence times may not need as frequent channel
adaptations as network environments with shorter channel coherence times. Thus the
channel adaptation information for longer channel coherence times may be exchanged
on a proportionately less frequent basis.
[0009] Conversely, networks in highly dynamic environments may need to exchange adaptation
information more often in order to maximize the efficiency of the channel adaptations.
A method, system and/or technique for efficient link adaptation between communicating
devices is needed.
[0010] The document
WO03/028323 discloses a wireless network in which the separation between the pilots within a
data burst is varied according to the coherence time of the channel.
BRIEF DESCRIPTION OF THE DRAWING.
[0011] Aspects, features and advantages of the present invention will become apparent from
the following description of the invention in reference to the appended drawing in
which like numerals denote like elements and in which:
Fig. 1 is a block diagram of an exemplary communication system according to various
embodiments of the present invention;
Fig. 2 is a timing diagram showing various rates of channel sounding signals in proportion
to the channel coherence times in a communications network according to one example
embodiment of the present invention;
Fig. 3 is a sequence diagram showing a method of varying sounding signal rates according
to one embodiment of the present invention; and
Fig. 4 is a block diagram of a communication apparatus which uses varying sounding
signal rates according to one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION.
[0012] While the following detailed description may describe example embodiments of the
present invention in relation to wireless networks utilizing Orthogonal Frequency
Division Multiplexing (OFDM) adaptive modulation, the embodiments of present invention
are not limited thereto and, for example, can be implemented using other modulation
schemes which may utilize link adaptation information where suitably applicable.
[0013] The following inventive embodiments may be used in a variety of applications including
transmitters and receivers of a radio system, although the present invention is not
limited in this respect. Radio systems specifically included within the scope of the
present invention include, but are not limited to: wireless local area network (WLAN)
systems, wireless personal area networks (WPAN) systems, wide metropolitan area network
(WMAN) systems and wireless wide area network (WWAN) systems including network interface
devices and peripherals such as network interface cards (NICs), base stations, access
points (APs), gateways, bridges, hubs and cellular radiotelephones. Further, the radio
systems within the scope of the invention may include cellular radiotelephone systems,
satellite systems, personal communication systems (PCS), two-way radio systems, one-way
pages, two-way pagers, personal computers (PC), personal digital assistants (PDA),
personal computing accessories (PCA) and all existing and future arising systems which
may be related in nature and two which the principles of the invention could be suitably
applied.
[0014] Turning to Fig. 1, a wireless communication system 100 according to one of the embodiments
of the invention may include one or more user stations 110, 112 and one or more network
access stations 120. System 100 may be any type of wireless network such as a wireless
local area network (WLAN), wireless wide area network (WWAN) or cellular network where
stations 110, 112 communicate with access station 120 via a communication link or
channel. System 100 may further include one or more other wired or wireless network
devices as desired, for example basic services set (BSS), distribution system (DS)
and/or ad-hoc network components.
[0015] The communication channel conditions between stations 110, 112 and 120 may be measured
and/or estimated so that communications between these stations can be continually
adapted (if necessary) to facilitate efficient communications with reasonable quality.
[0016] In preferred embodiments system 100 is an adaptive OFDM network although the embodiments
of the invention are not limited in this respect. OFDM is the modulation currently
used in many wireless applications including the Institute of Electrical and Electronic
Engineers (IEEE) 802.11a and 802.11 g standards for WLANs. OFDM works by dividing
up a wideband channel into a larger number of sub-channels. By placing a subcarrier
in each sub-channel, each subcarrier may be modulated separately depending on the
signal to noise ratio (SNR) or other signal
characteristics in that particular narrow portion of the band. As the channel varies over time, adaptations
can be made on each subcarrier in order to continually optimize the data-carrying
capacity of the channel. This is referred to herein as "adaptive modulation." Alternate
and/or additional transmission parameters, such as subcarrier power allocation and/or
code rates, may also be adapted or modified to improve the efficiency of communications.
The various types of transmission adaptations are generically, individually and/or
collectively, referred to herein as "link adaptation" (LA).
[0017] Since the channel conditions are susceptible to change due to, for example, reflections,
interference, scattering or movement between stations, the channel conditions should
be continually evaluated so that transmission parameters can be modified to meet current
or recent channel conditions.
[0018] A relatively simple way to determine the channel conditions and/or whether a previous
link adaptation scheme has expired (i.e., lost its usefulness due to changes in the
channel) is to exchange training preambles and adaptation information between the
user station 110, 112 and the network access station 120 at every access. This approach
provides the freshest link adaptations but incurs a large overhead since it uses a
dedicated exchange between the stations to update the link adaptation.
[0019] Another approach is for user station 110, 112 to passively measure/estimate the channel
conditions in the downlink direction based on channel sounding signals broadcast from
access station 120. A channel sounding signal is a transmission which may be used
by proximate receivers to estimate current channel conditions whether or not they
are actively communicating with access station 120.
[0020] In example implementations of the present invention relating to WLAN, a channel sounding
signal might be a periodic access beacon transmission from an access point (AP) (e.g.,
network access station 120) or ad-hoc station. Access beacons, generically referred
to herein as AP beacons, are unsolicited broadcasts that are periodically repeated
so that proximate network stations (STAs) may detect the existence of and/or properties
of the network access station for acquisition purposes and/or link maintenance. In
one example, received AP beacon transmissions (i.e., in the downlink direction) can
be used by user stations 110, 112 to periodically sense and track the channel conditions
and/or identify any significant changes in the channel condition. User stations 110,
112 may then resynchronize with, and/or adapt subsequent transmission parameters to,
the access station 120 (i.e., in the uplink direction) based on channel conditions
of the detected AP beacons (i.e., in the downlink direction).
[0021] However, if the period between AP beacons (and/or other types of unsolicited broadcasts
such as communications between the AP and another STA) is longer than the channel
coherence time, the channel conditions estimated by user stations 110, 112 for the
last access beacon may no longer be valid for communications occurring a certain time
(e.g., the coherence time) after the last access beacon. Accordingly, in certain embodiments
of the present invention, the beacon rate of network access station 120 (or the interval
between periodic channel sounding signals) may be varied in proportion to a channel
coherence time in order to allow a user station 110 to passively detect changes in
the channel conditions. Once changes are detected, the user station 110 may adapt
future uplink communications to access station 120 and/or send training information
to access station 120 to facilitate its link adaptation.
[0022] The network access station may be adjusted to beacon at a slower or faster rate to
meet the link adaptation requirements of the network and preferably, the period between
beacons will not substantially exceed the channel coherence time. In this manner the
network access station and user station may exchange training information (i.e., update
the link adaptation scheme), only when the user station determines that the channel
conditions have changed from observing channel sounding signals in the downlink direction.
[0023] If the channel coherence time is relatively short, it may be inefficient to increase
the AP beacon rate too much since AP beacons may include additional overhead other
than just a training preamble. Such additional overhead may include information to
identify the network for example, service set identifier (SSID), supported rate/mode,
supported security mechanisms, etc. which may not be needed for link adaptation. In
cases with relatively shot channel coherence times, network access station 120 (e.
g. , an AP or ad-hoc station) may be configured to transmit a different type of channel
sounding signal (i.e., other than an AP beacon) so that an interval between any two
successive sounding signals docs not significantly exceed the channel coherence time.
[0024] These additional channel sounding signals are used in addition to the access beacons
so that the channel conditions between stations can be estimated and link adaptations
can be implemented (if necessary) in a period proportional to the channel coherence
time. In certain embodiments the additional channel sounding signals are low overhead
signal fragments, such as a training preamble without a data payload. In combination
with the access beacons, the time period between successive channel sounding signals
may be varied in accordance with the channel coherence time. This type of dynamic
adjustment allows user stations 110,112 to maintain adaptation coherence tracking
without actively sounding the channel and without incurring the full overhead of,
for example, the AP 120 transmitting closely spaced AP beacons.
[0025] Turning to Fig. 2, an example timing diagram 200 illustrates sample varying timing
sequences 210,220 and 230 for transmitting channel sounding signals according to various
embodiments of the present invention.
[0026] Timing sequence 210 demonstrates a transmitting unit (e.g. AP) in a network environment
with a long coherence time. The AP transmits beacons 212 at every time interval T1,
referred to as the beacon rate.
[0027] Interval T1 may preferably have a maximum length selected to conserve power and utilize
the minimum channel bandwidth but shorter than a coherence time of the channel.
[0028] Timing sequence 220 demonstrates beacons 212 being transmitted at shortened time
intervals T2. In this example, interval T2 is reduced (as compared with T1) to increase
the access beacon rate for a network environment having a shorter channel coherence
time as compared with the coherence time for timing sequence 210.
[0029] However, as previously mentioned, it may be undesirable to increase the access beacon
rate too much since; for example, an AP beacon may carry additional overhead other
than a training preamble. Timing sequence 230 demonstrates an embodiment for environments
with relatively short channel coherence times. Here, the network access station may
sound the channel using
AP beacons 212 which occur at some maximum rate (e. g. , every interval T2). However,
additional sounding fragments 232, having lower overhead (reflected by shorter arrows)
than typical AP beacons, are transmitted in the interval between beacons 212. This
embodiment allows the user stations to track channel conditions without the overhead
associated with frequent beacons.
[0030] Depending on the channel coherence time, the network access station may transmit
more than one sounding fragment 232 between each access beacon 212, and such that
a time interval T3 between any two successive transmissions (e.g., between beacon
and sounding fragment or two sounding fragments) will not substantially exceed the
coherence time of the channel.
[0031] Turning to Fig. 3, a method 300 of communicating in an adaptive link wireless network
according to certain embodiments of the invention generally includes wireless devices
exchanging 305 (e.g., at least one device sending information to the other) training
information to establish a communication channel in a wireless network. In one embodiment,
one or both of the wireless devices send training symbols or pilot signals to the
other in an attempt to synchronize and establish a coherent adaptive OFDM communication
channel.
[0032] The devices may receive the training information and estimate the characteristics
of the channel for adapting OFDM transmission parameters, including determining 310
the channel coherence time. Once the channel coherence time is known/estimated, one
of the wireless devices (e.g., an AP) can then periodically broadcast 315 channel
sounding signals (e.g., an AP beacon, low-overhead signal fragments or combination
thereof) at intervals proportionate to the channel coherence time. The device not
transmitting channel sounding signals, may then periodically receive each channel
sounding signal and estimate 320 the channel conditions to passively determine 325
if any significant changes have occurred in the channel.
[0033] If any significant changes in the channel are identified, one or both of the devices
may then exchange 330 additional training information so that both devices may update,
if necessary, their adaptive OFDM transmission parameters (e.g., modulation scheme,
power allocation, etc.) in accordance with the new channel conditions.
[0034] Turning to Fig. 4, an example network apparatus 400 which may implement the various
embodiments of the present invention generally includes a radio frequency (RF) interface
410 and a baseband and medium access controller (MAC) processor portion 450.
[0035] RF interface 410 may be any component or combination of components operative to send
and receive multi-carrier modulated signals. In one example RF interface includes
a receiver 412, transmitter 414 and frequency synthesizer 416. Interface 410 may also
include bias controls and a crystal oscillator and/or one or more antennas 418. Furthermore,
RF interface 410 may alternatively or additionally use external voltage-controlled
oscillators (VCOs), surface acoustic wave filters, IF filters and/or RF filters. Various
RF interface designs and their operation are known in the art and the description
thereof is therefore omitted.
[0036] In preferred embodiments interface 410 is configured to be compatible with one or
more of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 frequency
band standards for wireless local area networks (WLAN), however European or other
standards may also apply. Most preferably, interface 410 is configured for compatibility
and/or backward compatibility with the IEEE 802.11(a-b) (g) and/or (n) standards for
WLAN.
[0037] Baseband and MAC processing portion 450 communicates with RF interface 410 to process
receive/transmit signals and may include, by way of example only, an analog-to-digital
converter 452 for down converting received signals, a digital to analog converter
454 for up converting signals for transmission, a baseband processor 456 for physical
(PHY) layer processing of respective receive/transmit signals, and one or more memory
controllers 458 for managing read-write operations from one or more internal and/or
external memories (not shown). Processing portion 450 may also include processor 459
for medium access control (MAC)/data link layer processing. In certain embodiments
of the present invention, processor 459 or additional circuitry (not shown) may be
configured to perform the processes for identifying channel coherence time, adjusting
the rate of channel sounding signals and/or channel estimation (e.g., 310, 315, or
320; Fig. 3). Alternatively or in addition, baseband processor 456 may share processing
for these functions or perform these processes independent of processor 459. MAC and
PHY processing may also be integrated into a single component if desired.
[0038] Apparatus 400 may be implemented as, for example, a user station (STA) or as an access
point (AP) described previously and the functions and/or specific configurations of
apparatus 400 would be suitably selected or omitted.
[0039] The components and features of apparatus 400 may be implemented using any combination
of discrete circuitry, application specific integrated circuits, logic gates and/or
single chip architectures. Further, the features of apparatus 400 may be implemented
using microcontrollers, programmable logic arrays and/or microprocessors or any combination
of the foregoing where suitably appropriate.
[0040] It should be appreciated that the example apparatus 400 shown in the block diagram
of Fig. 4 is only one functionally descriptive example of many potential implementations
and that division, omission or inclusion of block functions in Fig. 4 does not infer
that the hardware components, circuits and/or elements for implementing these functions
would be divided, omitted, or included in embodiments of the present invention.
[0041] Embodiments of the present invention may be implemented using single input single
output (SISO) systems, multiple input multiple output (MIMO) systems or any combination
thereof. Further, embodiments of the invention may utilize multi-carrier code division
multiplexing (MC-CDMA) multi-carrier direct sequence code division multiplexing (MC-DS-CDMA)
or any other existing or future arising modulation or multiplexing scheme compatible
with the features of the present invention. Unless contrary to physical possibility,
the inventors envision the methods described herein: (i) may be performed in any sequence
and/or in any combination; and (ii) the components of respective embodiments combined
in any manner.
[0042] Although there have been described preferred embodiments of this novel invention,
many variations and modifications are possible without departing from the scope of
the invention and the embodiments described herein are not limited by the specific
disclosure above, but rather should be limited only by the scope of the appended claims.
1. A method (300) of communicating between an access point (120) and a network station
(110, 112) in a channel of a link adaptive wireless network, the method comprising,
at the access point (120):
broadcasting an access beacon (212) at predetermined intervals (T1;T2);
transmitting (315) one or more sounding fragments (232) between said broadcasted access
beacons (212), said one or more sounding fragments comprising a training preamble
with no payload, for use by the receiving network station to evaluate a condition
of the channel between the access point and the network station, wherein said one
or more sounding fragments (232) are transmitted at determined intervals proportionate
to a coherence time of the channel; and
adjusting the rate of transmission of the access beacons if it is determined by the
receiving network station (110,112) that changes have occurred to the condition of
the channel.
2. The method (300) of claim 1, wherein communicating between the first station and the
second station in the link adaptive wireless network comprises using orthogonal frequency
division multiplexing (OFDM) with adaptive bit modulation.
3. The method (300) of claim 1, wherein communicating between the first station and the
second station in the link adaptive wireless network comprises using OFDM with adaptive
subcarrier power loading.
4. An access point (120) for communicating with a network station (110, 112) in a channel
of a link adaptive wireless network, comprising:
a transmitter (414); and
a control unit (459) communicatively coupled to the transmitter and operative to control
the transmitter to:
broadcast an access beacon (212) at predetermined intervals (T1;T2); and
transmit one or more sounding fragments (232) between said broadcasted access beacons
(212), said one or more sounding fragments comprising a training preamble with no
payload, for use by the receiving network station to evaluate a condition of the channel
between the access point and the network station, wherein said one or more sounding
fragments (232) are transmitted at determined intervals proportionate to the coherence
time of the channel; and
the control unit (459) further including means for adjusting the rate of transmission
of the access beacons when it is determined by the receiving network station (110,112)
that changes have occurred to the condition of the channel.
5. The system (100) of claim 4 wherein the transmitter (414) transmits orthogonal frequency
division multiplexing (OFDM) signals.
6. The access point of claim 4 further comprising an antenna (418) coupled to the transmitter
(414) and operative to broadcast multi-carrier signals.
7. A wireless communication system comprising:
an access point (120) according to any one of claims 4 to 6; and
a network station (110,112) for communicating with the access point (120) in a channel
of a link adaptive wireless network, the network station (110,112) including:
a channel estimator configured to estimate (320) a condition of the channel based
on received channel sounding signals (212) periodically broadcast (315) by the access
point (120) at an interval corresponding to a coherence time of the communication
channel and one or more channel sounding fragments (232) having lower overhead than
the channel sounding signals between the intervals of the channel sounding signals;
and
a control unit configured to adapt communications with the network station according
to detected changes in the condition of the communication channel.
8. The device of claim 7 wherein the network station comprises a wireless local area
(WLAN) station (STA).
9. The device of claim 7 wherein the access point (120) and the network station (110,112)
communicate using orthogonal frequency division multiplexing (OFDM) signals.
10. The device of claim 9 further comprising an antenna configured to broadcast and receive
the OFDM signals.
11. The device of claim 9 further comprising multiple antennas configured to broadcast
and receive the OFDM signals.
1. Verfahren (300) für die Kommunikation zwischen einem Zugangspunkt (120) und einer
Netzwerkstation (110, 112) in einem Kanal eines drahtlosen Netzwerks mit Verbindungsanpassung,
wobei das Verfahren am Zugangspunkt (120) umfasst:
das Senden eines Zugangsfunksignals (212) in vorbestimmten Intervallen (T1;T2);
das Übertragen (315) eines oder mehrerer Tonfragmente (232) zwischen den gesendeten
Zugangsfunksignalen (212), wobei das eine oder die mehreren Tonfragmente eine Trainingspräambel
ohne Nutzlast umfassen, die von der empfangenden Netzwerkstation verwendet wird, um
eine Bedingung des Kanals zwischen dem Zugangspunkt und der Netzwerkstation auszuwerten,
wobei das eine oder die mehreren Tonfragmente (232) in bestimmten Intervallen entsprechend
einer Kohärenzzeit des Kanals übertragen werden; und
das Anpassen der Übertragungsrate der Zugangsfunksignale, wenn von der empfangenden
Netzwerkstation (110,112) festgelegt wird, dass an der Bedingung des Kanals Änderungen
aufgetreten sind.
2. Verfahren (300) nach Anspruch 1, wobei das Kommunizieren zwischen der ersten Station
und der zweiten Station in dem drahtlosen Netzwerk mit Verbindungsanpassung das Verwenden
eines orthogonalen Frequenzmultiplexverfahrens (OFDM) mit adaptiver Bit-Modulation
umfasst.
3. Verfahren (300) nach Anspruch 1, wobei das Kommunizieren zwischen der ersten Station
und der zweiten Station in dem drahtlosen Netzwerk mit Verbindungsanpassung das Verwenden
von OFDM mit adaptiver Unterträger-Leistungsbelastung umfasst.
4. Zugangspunkt (120) für die Kommunikation mit einer Netzwerkstation (110, 112) in einem
Kanal eines drahtlosen Netzwerks mit Verbindungsanpassung umfassend:
ein Übertragungsgerät (414); und
eine Steuereinheit (459), die kommunikativ mit dem Übertragungsgerät verbunden und
betriebsfähig ist, um das Übertragungsgerät zu steuern, um:
ein Zugangsfunksignal (212) in vorbestimmten Intervallen (T1;T2) zu senden; und
eines oder mehrerer Tonfragmente (232) zwischen den gesendeten Zugangsfunksignalen
(212) zu übertragen, wobei das eine oder die mehreren Tonfragmente eine Trainingspräambel
ohne Nutzlast umfassen, die von der empfangenden Netzwerkstation verwendet wird, um
eine Bedingung des Kanals zwischen dem Zugangspunkt und der Netzwerkstation auszuwerten,
wobei das eine oder die mehreren Tonfragmente (232) in bestimmten Intervallen entsprechend
einer Kohärenzzeit des Kanals übertragen werden; und
wobei die Steuereinheit (459) ferner Mittel zum Anpassen der Übertragungsrate der
Zugangsfunksignale umfasst, wenn von der empfangenden Netzwerkstation (110,112) festgelegt
wird, dass an der Bedingung des Kanals Änderungen aufgetreten sind.
5. System (100) nach Anspruch 4, wobei das Übertragungsgerät (414) OFDM-Signale sendet.
6. Zugangspunkt nach Anspruch 4 ferner umfassend eine Antenne (418), die mit dem Übertragungsgerät
(414) verbunden und betriebsfähig ist, um Multiträgersignale zu senden.
7. Drahtloses Kommunikationssystem umfassend:
einen Zugangspunkt (120) nach einem der Ansprüche 4 bis 6; und
eine Netzwerkstation (110,112) für die Kommunikation mit dem Zugangspunkt (120) in
einem Kanal eines drahtlosen Netzwerks mit Verbindungsanpassung, wobei die Netzwerkstation
(110, 112) umfasst:
einen Kanalschätzer, der konfiguriert ist, um eine Bedingung des Kanals basierend
auf den empfangenen Kanaltonsignalen (212) zu schätzen (320), die periodisch durch
den Zugangspunkt (120) in einem Intervall, das einer Kohärenzzeit des Kommunikationskanals
entspricht, gesendet (315) werden, und wobei eines oder mehrere der Kanaltonfragmente
(232) einen geringeren Aufwand haben als die Kanaltonsignale zwischen den Intervallen
der Kanaltonsignale; und
eine Steuereinheit, die konfiguriert ist, um Kommunikationen mit der Netzwerkstation
gemäß ermittelter Änderungen in der Bedingung des Kommunikationskanals anzupassen.
8. Vorrichtung nach Anspruch 7, wobei die Netzwerkstation eine Wireless Local Area (WLAN)
Station (STA) umfasst.
9. Vorrichtung nach Anspruch 7, wobei der Zugangspunkt (120) und die Netzwerkstation
(110,112) mithilfe von OFDM-Signalen kommunizieren.
10. Vorrichtung nach Anspruch 9 ferner umfassend eine Antenne, die zum Senden und Empfangen
der OFDM-Signale konfiguriert ist.
11. Vorrichtung nach Anspruch 9 ferner umfassend mehrere Antennen, die zum Senden und
Empfangen der OFDM-Signale konfiguriert sind.
1. Procédé (300) de communication entre un point d'accès (120) et une station de réseau
(110, 112) dans un canal d'un réseau sans fil à adaptation de liaison, le procédé
comprenant, au point d'accès (120), les étapes consistant à :
- diffuser une balise d'accès (212) à des intervalles prédéterminés (T1 ; T2) ;
- transmettre (315) un ou plusieurs fragments de sondage (232) entre lesdites balises
d'accès diffusées (212), lesdits un ou plusieurs fragments de sondage comprenant un
préambule de formation avec aucune charge utile, à utiliser par la station de réseau
de réception pour évaluer un état du canal entre le point d'accès et la station de
réseau, dans lequel lesdits un ou plusieurs fragments de sondage (232) sont transmis
à des intervalles déterminés proportionnels à un temps de cohérence du canal ; et
- ajuster le taux de transmission des balises d'accès s'il est déterminé par la station
de réseau de réception (110, 112) que des changements sont survenus dans l'état du
canal.
2. Procédé (300) selon la revendication 1, dans lequel la communication entre la première
station et la deuxième station de réseau sans fil à adaptation de liaison comprend
l'utilisation d'un multiplexage à division de fréquence orthogonale (OFDM) avec la
modulation binaire adaptative.
3. Procédé (300) selon la revendication 1, dans lequel la communication entre la première
station et la deuxième station dans le réseau sans fil à adaptation de liaison comprend
l'utilisation d'OFDM avec une charge de puissance de sous-porteuse adaptative.
4. Point d'accès (120) pour communiquer avec une station de réseau (110, 112) dans un
canal d'un réseau sans fil à adaptation de liaison, comprenant :
- un émetteur (414) ; et
- une unité de commande (459) couplée de manière communicative à l'émetteur et opérationnelle
pour commander l'émetteur pour :
-- diffuser une balise d'accès (212) à des intervalles prédéterminés (T1 ; T2) ; et
-- transmettre un ou plusieurs fragments de sondage (232) entre lesdites balises d'accès
diffusées (212), lesdits un ou plusieurs fragments de sondage comprenant un préambule
de formation avec aucune charge utile, à utiliser par la station de réseau de réception
pour évaluer un état du canal entre le point d'accès et la station de réseau, dans
lequel lesdits un ou plusieurs fragments de sondage (232) sont transmis à des intervalles
déterminés proportionnels au temps de cohérence du canal ; et
-- l'unité de commande (459) comprenant en outre des moyens pour ajuster le taux de
transmission des balises d'accès s'il est déterminé par la station de réseau de réception
(110, 112) que des changements sont survenus dans l'état du canal.
5. Système (100) selon la revendication 4, dans lequel l'émetteur (414) transmet des
signaux de multiplexage à division de fréquence orthogonale (OFDM).
6. Point d'accès selon la revendication 4, comprenant en outre une antenne (418) couplée
à l'émetteur (414) et opérationnelle pour diffuser des signaux de multi-porteuses.
7. Système de communication sans fil comprenant :
- un point d'accès (120) selon l'une quelconque des revendications 4 à 6 ; et
- une station de réseau (110, 112) pour communiquer avec le point d'accès (120) dans
un canal d'un réseau sans fil à adaptation de liaison, la station de réseau (110,
112) comprenant :
-- un estimateur de canal configuré pour estimer (320) un état du canal sur la base
des signaux de sondage de canal reçus (212) périodiquement diffusés (315) par le point
d'accès (120) à un intervalle correspondant à un temps de cohérence du canal de communication
et un ou plusieurs fragments de sondage de canal (232) ayant une charge de transport
inférieure à celle des signaux de sondage de canal entre les intervalles des signaux
de sondage de canal ; et
une unité de commande configurée pour adapter les communications avec la station de
réseau en fonction des changements détectés de l'état du canal de communication.
8. Dispositif selon la revendication 7, dans lequel la station de réseau comprend une
station (STA) de réseau local sans fil (WLAN).
9. Dispositif selon la revendication 7, dans lequel le point d'accès (120) et la station
de réseau (110, 112) communiquent en utilisant des signaux de multiplexage à division
de fréquence orthogonale (OFDM).
10. Dispositif selon la revendication 9, comprenant en outre une antenne configurée pour
diffuser et recevoir les signaux OFDM.
11. Dispositif selon la revendication 9, comprenant en outre de multiples antennes configurées
pour diffuser et recevoir les signaux OFDM.